(1) Field of the Invention
The present invention relates to a digital radio system in which data are transmitted in bursts by Time Division Multiple Access (TDMA), Time Division Duplex (TDD) or packets, and more particularly, to an antenna switched diversity receiver.
(2) Related Arts
In a radio system, a transmission channel over radiowave propagation paths is subject to various environmental disturbances. Radiowave signals traveling through propagation paths are subject to "fading" such that a radiowave becomes weaker or transmission characteristics vary at a receiving point. For example, "multipath-fading" occurs when incoming waves, each having their respective phases and amplitudes, interfere with each other at a receiving point due to reflections or diffractions from obstacles along the transmission paths.
The multipath-fading is critical in a mobile radio system because it causes considerable fluctuation in received signal strength (electric field strength of a radio wave) each time a mobile station moves. Similarly, in a digital radio system, the multipath-fading causes an output decrease or waveform distortion, resulting in a high bit-error-rate (BER). BER represents a ratio of erroneously received bits to total bits in a received signal.
Given these problems, "diversity technique" has been proposed in a variety of embodiments. Basically, two or more branches having a small cross-correlation, i.e., a small probability of simultaneous fading, are used to reduce the fading effects by selecting or combining the output signals from the branches.
The diversity technique divides into several schemes, which include "space diversity", depending on the branches used. [For further information, see "Mobile Communications Engineering", William C. Y. Lee, McGraw-Hill Book Company] The digital radio system, which uses a single carrier, employs the space diversity: two or more transmission paths are constructed by setting physically separated antennas (usually spaced out about a carrier's wave length or more) in a matching number, making a cross-correlation small.
Further, the space diversity sub-divides into "combining diversity" and "selective-combining diversity". Two or more received signals are combined in the combining diversity, while a received signal having the best quality is selected by means of switching in the switching diversity. Compared with the combining diversity, the selective-combining diversity enables a more compact and economical receiver. This is because the combining diversity essentially includes a phase control circuit while the selective-combining diversity does not. However, on the other hand, abrupt changes in phases and amplitudes after every selection or switching and waveform distortion occur with the selective-combining diversity.
The selective-combining diversity further sub-divides into "antenna selection diversity" and "antenna switched diversity"; an example of the former is disclosed in Japanese Laid-open Patent Application No. 58-38037 and that of the latter in Japanese Laid-open Patent Application No. 56-68037. In the antenna selection diversity, each antenna is connected to their respective receiving units, and one antenna having the greatest received-signal-strength indicator (RSSI) is selected. Whereas in the antenna switched diversity, all antennas are connected to a single receiving unit, and they are switched one from another when the RSSI of a currently selected antenna falls below a predetermined threshold level.
More detailed explanation of the antenna selection diversity and antenna switched diversity will be given.
FIG. 1 is a block diagram depicting a structure of a conventional antenna selection diversity receiver. The receiver includes two physically separated antennas 11, 12. Signals received by the antennas 11, 12 are respectively inputted into receiving units 13, 14, to be decoded. The decoded signals are inputted into RSSI checking units 16, 17, respectively, so that either one having a greater RSSI is selected by a switching unit 15 under the control of a control circuit 18. The decoded signal thus selected is further outputted to an external device via a data output terminal 19.
When the cross-correlation between the antennas 11, 12 is small, even if one of the two signals fades out, still the other remains unfaded. Therefore, the receiver constructed as above always selects a signal having a greater RSSI, improving the quality of the decoded signal significantly. However, on the other hand, its inherent structure, i.e., including two or more branches and RSSI checking units, makes it difficult to realize a downsized or economical receiver.
FIG. 2 is a block diagram depicting a structure of a conventional antenna switched diversity receiver. The receiver includes two physically separated antennas 21, 22. Unlike the antenna selection diversity, one of the signals from the antennas 21, 22 is selected by a switching unit 23 first, and then, the selected signal is inputted into a receiving unit 24 to be decoded, and further outputted to an external unit via a data output terminal 27. The receiving unit 24 also outputs the selected signal to a RSSI checking unit 25, which compares the RSSI of the selected signal with a predetermined threshold: when the RSSI falls below the threshold, the switching unit 23 switches to the other currently non-selected antenna under the control of a control circuit 26.
Like in the antenna selection diversity, when the cross-correlation between the antennas 21, 22 is small, even if one of the two signals fades out, the other still remains unfaded in most of the cases. Thus, the quality of the decoded signal is significantly improved. Moreover, the structure including a set of one receiving unit and one RSSI checking unit realizes a more compact and economical receiver compared with the antenna selection diversity.
However, the antenna switched diversity is inferior to the antenna selection diversity in diversity effect. The diversity effect around the threshold is substantially the same since both the schemes determine an antenna based on the RSSI. However, when the mean RSSI is far in excess of the threshold, although an antenna with a greater RSSI will always be selected with the antenna selection diversity receiver, no switching will be carried out with the antenna switched diversity, effecting no diversity at all. There also may be a case when the mean RSSI is far below the threshold. On simultaneous fade-out, each antenna's RSSI stays lower than the threshold, causing an excessive switching, so-called "hunting", that generates unfavorable switching noise. Moreover, the diversity effect is considerably reduced if the receiving unit's transient response is slow.
In addition, an antenna switched diversity is easily affected by external noises or interferences, which include co-channel or adjacent channel interferences in a multi-cell system. Those noises or interferences increase the measured RSSI. Therefore, the setting of threshold level to switch becomes inappropriate and causes performance deterioration for antenna switched diversity, while, for antenna selection diversity, the performance is not affected because the measured RSSIs in both antennas are relatively compared each other.
For further understanding, the antenna switched diversity characteristics will be explained more in detail.
A variety of embodiments, such as switch-and-examine (SE) and switch-and-stay (SS), have been proposed depending on conditions for switch-activation or algorithms.
In the SE, the switching is repeated until the RSSI of one of the antennas exceeds a threshold. This may cause the hunting, and thus is not preferable, particularly in analog transmission. Whereas in the SS, even if a newly switched antenna's RSSI is below the threshold, the switching will be restarted after its RSSI exceeds the threshold. [For further information, see "Performance of Feedback and Switch Space Diversity 900 MHz FM Mobile Radio Systems with Rayleigh Fading" A. J. Rustako, Jr, Y. S. Yeh, R. R. Murray, pages 1257-1268, IEEE Transactions on Communications, Vol. Com-21, No. 11, November 1973]
Next, the analysis of the received signal's cumulative probability distribution (CPD) after switching and diversity gain will be given. The CPD represents a probability P that a carrier-to-noise ratio (CNR) .gamma. is equal or less than x, (.gamma..ltoreq.x). Assume that an antenna switched diversity receiver used herein includes two antennas and employs either the SS or SE using one threshold; the antennas have a small cross-correlation and both are subject to Rayleigh fading with a mean CNR .GAMMA.. Rayleigh fading is typically observed in land mobile transmission, which causes an abrupt RSSI fluctuation.
Since no-diversity CPD q.sub.x is expressed as: EQU q.sub.x =1-e.sup.-x/.GAMMA. ( 1)
P (.gamma..ltoreq.x) can be expressed as: EQU P (.gamma..ltoreq.x)=q.sub.x -q.sub..gamma..sbsb..GAMMA. +q.sub.x .multidot.q.sub..gamma..sbsb..GAMMA., when x&gt;.gamma..sub.T =q.sub.x .multidot.q.sub..gamma..sbsb..GAMMA., when x.ltoreq..gamma..sub.T( 2)
wherein .gamma..sub.T is a threshold CNR for switching.
A graph in FIG. 3 shows the result where .gamma..sub.T is -6.5, -4.0, -2.8 dB from the mean CNR .GAMMA.. In FIG. 3, the CPD characteristics of the no-diversity and selection diversity are also shown in dotted and dashed lines respectively for comparison.
To evaluate the diversity gain, a marginal quality in outage rate must be defined. The outage referred to herein is a state where the quality of a received signal degrades; i.e., bit errors occur because of a received signal drop due to fading. Let the marginal outage rate be 10% for the use of explanation herein.
The comparison between the dotted and dashed lines of cumulative probability reveals that the diversity gain in the selection diversity is large where a mean RSSI is sufficiently high (for example, at the center of a radio coverage), and small where the mean RSSI is low (for example, at the boundary of the coverage). In contrast, the diversity gain in the antenna switched diversity is large around a threshold level, and decreases rapidly above the threshold level. Note that the diversity gain of both diversities are equal at the threshold level.
More precisely, given the outage rate of 10%, the selection diversity obtains 5.5 dB as the diversity gain. Thus, to obtain the same diversity gain while keeping degradation within 1.5 dB (=diversity gain of 4 dB) in the antenna switched diversity, .gamma..sub.T must be set at a range of -6.5 dB to -2.8 dB from an optimum threshold level of -4 dB. This means that the threshold level must be set precisely within a range of -2.5 dB to +1.2 dB from the optimum level.
Next, more practically for a digital system, a mean BER is analyzed; the signal is modulated by non-coherent FSK (Frequency Shift Keying) herein. [For further information, see "Comparison of Selection and Switched Diversity Systems for Error-rate Reduction at Base-station Sites in Digital Mobile Radio Systems", J. D. Parsons, M. T. Feeney, pages 393-398, IEEE VTC'87, 1987.
A relation between a CNR .gamma. and a BER P.sub.e (.gamma.) in a non-fading environment is expressed as: EQU P.sub.e (.gamma.)=(1/2)e.sup.-.gamma./2 ( 3)
Then, a mean BER, P.sub.e.k, is expressed as: ##EQU1##
where P.sub.2 (.gamma.) is a probability density function of a CNR of a received signal.
Hence, a mean BER P.sub.e.1 in no-diversity is expressed as: EQU P.sub.e.1 =1/(2+.gamma..sub.o) (5)
where .gamma..sub.o is a mean CNR.
Therefore, a mean BER, P.sub.e.2, is expressed as: EQU P.sub.e.2 =P.sub.e.1 {1-e.sup..gamma..sbsp..GAMMA..sup./.gamma..sbsp.o (1-e.sup.-.gamma..sbsp..GAMMA..sup./2)} (6)
Expression (6) is differentiated to find a minimal condition for P.sub.e.2, then we get EQU .gamma..sub.T =2 ln(1+.gamma..sub.o /2) (7)
Thus, an optimum threshold CNR is determined by Expression (7).
However, since .gamma..sub.o is the mean CNR, in practice, it is difficult to accurately determine the optimum threshold CNR with Equation (7). Because it takes quite a long time to estimate the mean value for a slow moving speed, and the CNR must take into account external noise and inteference signals, which are difficult to be measured readily. When these external noise and interferences are not negligible, a total noise power is not easily estimated; therefore, it is almost impossible to estimate the CNR, although the RSSI can be measured.
A relationship between a threshold CNR and the mean BER is shown by a graph in FIG. 4. The dotted and dashed lines represent cases of the no-diversity and selection diversity, respectively. The graph shows that the optimum thresholds .gamma..sub.T for the mean CNRs 13 dB and 20 dB are 7 dB and 9 dB, respectively.
The comparison at troughs with the dashed and dotted lines reveals that the diversity gain in the antenna switched diversity at the mean CNR of 13 dB is reduced approximately by two-thirds compared with the selection diversity. Also the comparison reveals that the threshold CNR must be maintained precisely within a range of .+-.1.5 dB to prevent further reduction. Besides, the optimum threshold varies with the mean CNR out of this range (.+-.1.5 dB), and it is by no means easy to estimate the CNR when the external noise and interference signals are not negligible. This leads to a conclusion that setting a predetermined threshold is ineffective to upgrade the diversity effect in the case of the antenna switched diversity.
Thus, it can be concluded that the antenna selection diversity is advantageous over the antenna switched diversity in the diversity effect. Given these circumstances, a technique that realizes a compact, economical antenna switched diversity receiver with the diversity effect as excellent as the antenna selection diversity has been sought after.